Process for selective production of propylene from hydrocarbon fractions with four carbon atoms

ABSTRACT

For the selective production of propylene from an olefinic C 4  fraction, a process is implemented that successively comprises:  
     1) the selective hydrogenation of butadiene with isomerization of butene-1 into butene-2;  
     2) the separation by distillation of a mixture that is rich in isobutene and butene-1 at the top and a fraction that is rich in butene-2 at the bottom;  
     3) the skeletal isomerization of isobutene into n-butenes on the top fraction, with recycling in stage 1; and  
     4) the metathesis of the butene-2-rich fraction with ethylene.  
     The advantage of this process is to produce in a very selective way polymerization-quality propylene from all of the olefinic compounds of a C 4  fraction, including isobutene.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This is a divisional of application Ser. No. 09/846,690 filed May2, 2001. It is also related to application Ser. No. 09/745,722 filedDec. 26, 2000.

[0002] The invention relates to a process for selective production ofpolymerization-quality propylene from an olefinic C₄ fraction.

[0003] The steam-cracking of feedstocks that consist of light paraffinicfractions produces the ethylene and the propylene that are necessary topetrochemistry. It also produces a certain number of other heavierproducts, and in particular a C₄ hydrocarbon fraction that containsmainly butadiene-1,3, isobutene, n-butenes and butanes, accompanied bytraces of acetylenic hydrocarbons.

[0004] The catalytic cracking of heavy hydrocarbon feedstocks produces,alongside gasoline and gasoil fractions that are the main products,lighter products, including a C₄ hydrocarbon fraction that containsmainly isobutane, isobutene, n-butenes and butanes, accompanied by smallamounts of butadiene-1,3 and acetylenic hydrocarbons.

[0005] Until recently, only butadiene-1,3 and isobutene were used in thepolymer industry, in particular in the tire industry. The increase ofthe longevity of tires and a relative stagnation of the demand ensurethat there is now excess butadiene that is not used or is poorly used.To date, isobutene was used, for example, for the synthesis of etherswith the use of additives in automobile fuels or as a monomer in thesynthesis of polyisobutene. These uses, however, can lead to saturationand render the isobutene useless.

[0006] This invention proposes a process for treatment of a C₄hydrocarbon fraction that contains primarily isobutene, n-butenes,butanes, and butadiene-1,3 in a variable amount that includes theskeletal isomerization of isobutene into n-butenes and that makes itpossible to transform all of the C₄ unsaturated compounds into propylenethat can be used for, for example, polymerization.

[0007] The fractions that are treated in the process according to theinvention correspond to the C₄ fractions of conversion processes. Theycan correspond to, for example, the crude C₄ fraction forsteam-cracking, the C₄ fraction for steam-cracking after extraction ofthe butadiene that is commonly called raffinate-1, or the C₄ fractionfor catalytic cracking.

[0008] The relative proportions of ethylene and propylene that areproduced in a steam-cracking operation can be modulated to a certainextent by changing the nature of the feedstock and by modifying theoperating conditions (the degree of rigor) of the cracking. Theoperating method that is oriented toward a larger proportion ofpropylene, however, inevitably entails a decline in the yield ofethylene and a higher C₄ fraction and gasoline fraction production.

[0009] Another object of this invention is thus to increase thepropylene production while maintaining a high ethylene yield with thetreatment of the C₄ hydrocarbon fraction and therefore without it beingnecessary to reduce the rigorous conditions of the steam-crackingdevice.

[0010] The process that is the object of the invention is morespecifically a process for converting into propylene an olefinic C₄fraction, whereby said fraction comprises diolefins, primarilybutadiene-1,3, butene-1, butene-2, isobutene and acetylenic impurities,and whereby said process comprises the following stages that take placesuccessively:

[0011] 1) the selective hydrogenation of diolefins and acetylenicimpurities with isomerization of butene-1 into butenes-2, carried out ina reactor, in the presence of a catalyst, in order to obtain an effluentthat contains for the most part butenes-2 and isobutene, and thatcontains virtually no diolefins or acetylenic compounds;

[0012] 2) the separation by distillation of a top fraction that containsfor the most part isobutene and unconverted butene-1 in the first stage,and a bottom fraction that contains essentially butene-2 and butane; and

[0013] 4) the metathesis of the butenes-2 fraction that is obtained fromstage 2 with the ethylene so as to obtain an effluent that containspropylene, whereby the metathesis is followed by a separation of thepropylene;

[0014] whereby said process also comprises a stage 3 of skeletalisomerization of the isobutene into n-butenes in the top fraction, withrecycling of at least a portion of the effluent in stage 1.

[0015] The isomerization of butene-1 into butenes-2 as carried out instage 1 can also be carried out in part in association with thedistillation (stage 2) by using an isomerization catalyst as describedfor stage 1 according to the teachings of French FR-B-2 755 130, in thename of the applicant.

[0016] The special conditions of the different stages of the processaccording to the invention, carried out from a C₄ hydrocarbon fractionthat contains primarily isobutene, n-butenes, butanes, as well asbutadiene in a variable amount, whereby said C₄ fraction is subjected tothese stages to produce essentially propylene, are described in moredetail below.

[0017] The main object of the first stage is to transform the butadieneand the n-butenes into butenes-2. Actually, the butenes-2 are the sourceof the propylene that is produced in stage 4 of metathesis in thepresence of ethylene. It is therefore desirable to maximize thebutenes-2 yield, i.e., to draw as close as possible to the ratio that isallowed by thermodynamics. The second object of this stage is toeliminate the acetylenic hydrocarbon traces that are always present inthese fractions and that are poisons or contaminants for the subsequentstages.

[0018] In this first stage, the following reactions are thus carried outsimultaneously in the presence of hydrogen:

[0019] the selective hydrogenation of butadiene into a mixture ofn-butenes;

[0020] the isomerization of butene-1 into butenes-2 to obtain adistribution that is close to the thermodynamic equilibrium; and

[0021] the selective hydrogenation of the acetylenic hydrocarbon tracesinto butenes.

[0022] These reactions can be carried out with various specificcatalysts that comprise one or more metals, for example from group 10 ofthe periodic table (Ni, Pd or Pt), deposited on a substrate. A catalystthat comprises at least one palladium compound that is fixed on arefractory mineral substrate, for example on an alumina, is preferablyused. The palladium content in the substrate can be 0.01 to 5% byweight, preferably 0.05 to 1% by weight. Various pretreatment methodsthat are known to one skilled in the art optionally can be applied tothese catalysts to improve the selectivity in the hydrogenation ofbutadiene into butenes at the expense of the total hydrogenation ofbutane that it is necessary to avoid. The catalyst preferably contains0.05 to 10% by weight of sulfur. Advantageously, a catalyst is used thatcomprises palladium that is deposited on alumina, and sulfur.

[0023] The catalyst can advantageously be used according to the processthat is described in Patent FR-B-2 708 596. According to this process,the catalyst is treated, before it is loaded into the hydrogenationreactor, by at least one sulfur-containing compound that is diluted in asolvent, then the catalyst that is obtained that contains 0.05 to 10% byweight of sulfur is loaded into the reactor and activated under aneutral atmosphere or a reducing atmosphere at a temperature of 20 to300° C., a pressure of 0.1 to 5 MPa and a VVH of 50 to 600 h⁻1, and thefeedstock is brought into contact with said activated catalyst.

[0024] The use of the catalyst, preferably with palladium, is notcritical, but it is generally preferred to use at least one down-flowreactor through a catalyst fixed bed. When the proportion of butadienein the fraction is large, which is the case, for example, of asteam-cracking fraction when it is not desired to extract the butadienefrom it for specific uses, it may be advantageous to carry out thetransformation in two reactors in series to better monitor theselectivity of the hydrogenation. The second reactor can have a risingflow and play a finishing role.

[0025] In some cases, it may be advisable to dilute the feedstock thatis to be treated by said C₄ fraction in which the butadiene is partiallyor totally hydrogenated.

[0026] The amount of hydrogen that is necessary for all of the reactionsthat are carried out in this stage is adjusted based on the compositionof the fraction advantageously to have only a slight hydrogen excessrelative to the stoichiometry.

[0027] The operating conditions are selected such that the reagents andthe products are in liquid phase and such that they promote theformation of butenes-2. It may be advantageous, however, to select anoperating mode such that the products are partially evaporated at theoutlet of the reactor, which facilitates the thermal monitoring of thereaction. The temperature may vary from 0 to 200° C., preferably from 0to 150° C. or better from 0 to 70° C. The pressure may be adjusted to avalue of 0.1 to 5 MPa, preferably 0.5 to 4 MPa and advantageously from0.5 to 3 MPa, such that the reagents, at least in part, are in liquidphase. The volumetric flow rate may be from 0.5 to 20 h⁻1 and preferablyfrom 1 to 10 h⁻1, with an H₂/diolefin molar ratio of 0.5 to 5 andpreferably 1 to 3.

[0028] The hydroisomerization reactor or reactors may advantageously befollowed by a stabilization column that eliminates the traces of gaseoushydrocarbons that are optionally present in the feedstock hydrogen.

[0029] The object of the second stage is to separate by distillation theC₄ fraction that is obtained from the preceding stage to obtain, on theone hand, a fraction that contains isobutene, isobutane and the majorityof butene-1, on the other hand, a fraction that contains a small amountof butene-1, butenes-2 and n-butane. The isobutene that is thusconcentrated is conducted to stage 3 of skeletal isomerization. Thebutenes-2 fraction is directed toward the metathesis stage.

[0030] To reduce as much as possible the butene-1 concentration in theeffluent of the column head, it is possible to use a reactivedistillation column that comprises, on the inside of the column oroutside, one or more feedstocks of the catalyst that is used asdescribed for stage 1. The reactive distillation column that is used canthen be of any type. In a preferred arrangement, at least one zone thatcontains the catalyst is arranged. The mechanical arrangement of thecatalyst in the catalytic zone or zones should be such that it disturbsthe flows of vapor and liquid as little as possible between the twoseparation zones that frame it. The catalyst can be placed, for example,in a thin layer on perforated plates or on grids, or in bags that aresuspended or laid on substrates that ensure their mechanical behavior,or any other way that is known to one skilled in the art. On the otherhand, the catalyst can be placed in the column so that only an upwardflow of liquid phase passes through it. It can also be arranged in theform of catalytic packing according to the different implementationsthat are known. The separation zones that frame the catalytic zones cancomprise plates or packing.

[0031] One of the uses of the column can correspond to, for example, theone that is described in French Patent FR-B-2 755 130 in the name of theapplicant.

[0032] The distillation top fraction that is rich in isobutene issubjected in stage 3 to a skeletal isomerization that is intended totransform the isobutene into n-butenes, which can be sent to the inletof zone 1. The optionally present isobutene may be purged.

[0033] This skeletal-isomerization reaction can be carried out withcatalysts that have an alumina base or more particularly activated orvapor-treated aluminas (U.S. Pat. No. 3,558,733) or that comprisecompounds such as those of titanium (U.S. Pat. No. 5,321,195 of theapplicant) and/or boron (U.S. Pat. No. 5,659,104 of the applicant) inthe case of eta- or gamma-aluminas, halogenated aluminas (U.S. Pat. No.2,417,647) or bauxite. Zeolites or molecular sieves that have amono-dimensional microporous network (Patent Documents EP-A-523 838,EP-A-501 577 and EP-A-740 957 of the applicant) can also constituteactive phases of skeletal-isomerization catalysts. The alumina-basedcatalysts are generally used in the presence of water at temperatures offrom 200° C. to 700° C., at a pressure of 0.1 to 2 MPa, at a volumetricflow rate of 0.1 to 20 h⁻1 and with a molar ratio of injected water tohydrocarbon of 0.1 to 10. The zeolitic catalysts are used without water,at a temperature of 200° C. to 500° C., under a pressure of 0.1 to 2 MPaand at a volumetric flow rate of 0.1 to 20 h⁻1.

[0034] The skeletal isomerization of the isobutene into n-butenes iscarried out preferably with a catalyst that comprises alumina andtitanium at a temperature of 300° C. to 570° C., a pressure of 0.1 to 1MPa, at a volumetric flow rate of 0.1 to 10 h⁻1, and in the presence ofwater injection, whereby the molar ratio of injected water/olefinichydrocarbons is 0.1 to 10.

[0035] A catalyst that contains alumina and 0.03 to 0.6% by weight oftitanium and that can also contain 0.05 to 5% by weight of an oxide ofan element of group IIIA, whereby this element advantageously is boron,will preferably be used in the invention. Before being brought intocontact with the hydrocarbons of the feedstock, this catalystadvantageously will have undergone a water vapor treatment at atemperature of 120-700° C. under a partial water vapor pressure that isgreater than 0.05 MPa, for a period of 0.5 to 120 hours.

[0036] The bottom fraction of the distillation zone, rich in butenes-2,preferably contains at most 1% by weight of butene-1, advantageously atmost 0.5% by weight, and at most 1% by weight of isobutene. Thebutenes-2 fraction that is obtained from stage 2 does not containoutside contaminants and can therefore be sent directly into the fourthstage of the process. In this last stage, the butenes-2 are reacted withethylene to produce propylene by metathesis. Because of the small amountof butene-1 in the feedstock, the by-product formation is very limited.

[0037] The metathesis reaction of the ethylene with the butenes-2 can becatalyzed by varied metallic oxides that are deposited on substrates,for example, by molybdenum, tungsten or rhenium oxides. A catalyst thatcomprises at least one rhenium oxide that is deposited on a substratethat comprises a refractory oxide that itself contains at least aluminaand that has an acidic nature, such as, for example, alumina itself,silica-aluminas or zeolites, is preferably used.

[0038] It is possible to cite, by way of preferred examples, thecatalysts that comprise rhenium heptoxide that is deposited on agamma-alumina, such as those described in U.S. Pat. No. 4,795,734. Therhenium content (expressed in metallic rhenium) can be 0.01 to 20%,preferably 1 to 15% by weight. The catalysts are subjected to, forexample, a final thermal activation at a temperature of 400 to 1000° C.for a period of 10 minutes to 5 hours under a non-reducing atmosphere.

[0039] The catalysts that comprise rhenium heptoxide that is depositedon an alumina can also be modified by the addition of an oxide ofanother metal. Such modified catalysts comprise, for example, rhenium inthe oxide state, at a rate of 0.01 to 20% by weight expressed inmetallic rhenium, deposited on a substrate that contains at least 75% byweight of alumina and 0.01 to 30% by weight of at least one oxide of ametal that is selected from the group that is formed by niobium andtantalum, as described in Patent FR-B-2 709 125. Another class ofmodified catalysts comprises rhenium in the oxide state, at a rate of0.01 to 20% by weight expressed in metallic rhenium, deposited on asubstrate that contains at least 75% by weight of alumina and 0.01 to10% by weight of aluminum of a compound of formula (RO)_(q)AIR′_(r),where R is a hydrocarbyl radical of 1 to 40 carbon atoms, R′ is an alkylradical of 1 to 20 carbon atoms, and q and r are equal to 1 or 2, withq+r equal to 3 (see Patent FR-B-2 740 056).

[0040] The metathesis reaction is carried out preferably in a liquidphase, without oxygen, oxidized compounds and moisture, and at atemperature of 0 to 200° C., preferably 20 to 150° C., under a pressureat least equal to the vapor pressure of the reaction mixture at thereaction temperature.

[0041] The catalyst can be used in a fixed bed. Since it must beregenerated frequently, however, it is then necessary to use at leasttwo reactors in parallel, whereby one is in use while the other is beingregenerated. A catalyst moving bed system as described in French PatentFR-B-2 608 595 is preferably used. The catalyst is drawn off at regulartime intervals from the bottom of the reactor and transferred to acontinuous regeneration system, from where it is sent to the top of thereactor.

[0042] Taking into account the limitations that are imposed bythermodynamics, the unconverted ethylene is fractionated in a firstdistillation column and recycled in the metathesis reactor. A seconddistillation column separates the propylene and the unconverted C₄hydrocarbons that can be recycled in the metathesis reactor or inanother location of the process.

[0043] When the process is applied to a steam-cracking C₄ fraction, itmay be advantageous to integrate the metathesis unit with the crackingdevice to take advantage of the fractionation train of the latter. Theethylene that is obtained from the steam-cracking operation is then usedin the metathesis stage.

[0044] The succession of treatments adopted in the process according tothe invention has many advantages. The most reactive compounds of thefraction, in particular the butadiene-1,3 that is present in variableamounts, as well as the traces of acetylenic hydrocarbons, aretransformed from the first stage and therefore will not be the cause ofparasitic reactions in the following stages. Furthermore, the selectivehydrogenation of diolefins (butadiene-1,3 and, if necessary,butadiene-1,2) into butenes, the hydroisomerization of butene-1 and theskeletal isomerization of isobutene into n-butenes make it possible toincrease considerably the butenes-2 concentration in the fraction, whichthereby promotes a high yield of propylene in the metathesis stage.

[0045] The fractionation of the fraction that is obtained from thehydroisomerization into isobutene and butene-1, on the one hand, andinto butenes-2, on the other hand, makes it possible to concentrate theisobutene for the skeletal-isomerization stage, as well as the butenes-2that are then subjected to metathesis.

[0046] In addition, in the following metathesis stage (stage 4), the lowbutene-1 content in the butenes-2-rich fraction makes it possible toobtain a propylene selectivity that is close to 100%. Actually, it isknown that the butene-1 reacts with the butenes-2 to produce propyleneand pentenes, and that it reacts with itself to produce hexenes.Pentenes and hexenes are by-products of low value, which it is necessaryto eliminate, in a costly manner. The process therefore makes possiblean appreciable increase of the propylene yield and facilitates therecycling of butenes-2 in the metathesis reactor, since there are fewpentenes and hexenes to eliminate.

[0047] The invention also relates to an installation (illustrated byFIG. 1) that is used to implement the process that is described above.

[0048] It successively comprises:

[0049] a zone 1 for selective hydrogenation with isomerization ofbutene-1 into butene-2, whereby said zone comprises at least one means 1for introducing fraction C₄ that is to be converted, at least one means3 for the output of the effluent and at least one means 2 for theintroduction of hydrogen, whereby said zone also comprises at least onecatalyst bed;

[0050] a zone 2 for separation that comprises at least one means 3 forthe introduction of the effluent that is obtained from zone 1, at leastone means 5 for the output of isobutene and butene-1, at least one means4 for the output of butene-2 and n-butane; and

[0051] a zone 4 for metathesis that contains at least one catalyst bedand that comprises at least one means 4 for introducing the effluentthat is obtained from zone 2, at least one means 7 for introducingethylene and at least one means 8 for the output of the propylene,

[0052] whereby said installation also comprises a skeletal-isomerizationzone 3 that comprises at least one means 5 for introducing the effluentthat is obtained from zone 2, at least one means 6 for recycling fromthe outlet of zone 3 to the inlet of zone 1 and at least one means 9 forpurging optionally present isobutane, whereby said zone also comprisesat least one catalyst bed that preferably comprises alumina andtitanium.

[0053] In a particularly advantageous way, the C₄ fraction is obtainedfrom an upstream steam-cracking zone, whereby the means for introducingthe fraction that is to be converted into zone 1 is connected to saidsteam-cracking zone, and the means for introducing the ethylene intozone 4 is connected to said steam-cracking zone.

[0054] The following example illustrates the invention without limitingits scope.

EXAMPLE

[0055] A C₄ fraction at the outlet of the steam-cracking device has thecomposition that is indicated in Table 1 (flow 1). In this table, theabbreviations have the following meanings:

[0056] MAPD=methylacetylene+propadiene,

[0057] BBV=butadiene-1,2+butyne-1+vinylacetylene.

[0058] The C₄ fraction that is to be treated is first mixed with flow 6for recycling the effluent of zone 3 (skeletal isomerization), then itis subjected to a hydrogenation and hydroisomerization treatment inzone 1. It is introduced continuously, with the mass flow rate indicatedin Table 1, and under a pressure of 1.4 MPa, in a first reactor thatcomprises a fixed bed of a catalyst that consists of palladium onalumina that was sulfurized in advance. Hydrogen (mixed with methane) isalso injected into this reactor, as indicated in Table 1

[0059] (flow 1+6). The effluent of this first reactor is then treated ina finishing reactor that is loaded with the same catalyst. At the outlet(Table 1, flow 3), acetylenic compounds are removed from the fraction,and the butadiene was transformed essentially into butenes, which arefor the most part butenes-2, butene-1 having been isomerized. Thefraction is then treated in a stabilization column, where the residualhydrogen and the methane are separated.

[0060] In zone 2, the hydroisomerized C₄ fraction (effluent of zone 1)is subjected to a fractionation in a distillation column. This columncomprises about 90 plates and operates at a pressure of 0.7 MPa in thereflux flask so as to allow the use of cooling water in the topcondenser. The reflux rate is adjusted, on the one hand, to limit theloss of butene-2 in the distillate, and, on the other hand, to reducethe contents of butene-1 and isobutene in the bottom product to limit tothe maximum the formation of pentene and hexene by-products in thesubsequent metathesis stage. Top flow 5 from distillation is directedtoward skeletal-isomerization zone 3, and bottom flow 4 of distillationis directed toward metathesis zone 4. These two flows have thecomposition that is given in Table 1.

[0061] In zone 3, the catalyst that is used for theskeletal-isomerization reaction of the isobutene consists ofgamma-alumina that contains 0.1% by weight of titanium. It was subjectedto a treatment under water vapor at 560° C. for 20 hours with a partialwater vapor pressure that is equal to 0.08 MPa. It is used to isomerizethe isobutene that exits from zone 2 (flow 5) at a temperature of 500°C., a pressure of 0.1 MPa, a water/isobutene molar ratio that is equalto 2 and a volumetric flow rate of 1.3 h⁻1. Under these conditions, theconversion of isobutene is 57% by weight, and the n-butene selectivityis 90%. The effluent of zone 3 is separated into a recycling flow 6 tohydroisomerization zone 1 and a flow 9 that is a purge that is intendedto avoid the accumulation of the isobutane that is present in thefeedstock fraction. The composition of these two flows is provided inTable 1.

[0062] In zone 4, the bottom distillation fraction that contains mainlybutene-2 (flow 4) is reacted with ethylene (overall composition: flow4+7 of Table 1) in a metathesis catalyst that consists of rhenium oxideon gamma-alumina (8% by weight of metal rhenium), prepared according tothe teachings of U.S. Pat. No. 4,795,734. The C₄ fraction is mixed atthe inlet of the metathesis reactor with the make-up ethylene, as wellas with recycling flows of ethylene and butenes. This reactor operatesin a moving bed, as described in Patent FR-B-2 608 595, at a temperatureof 35° C. and under a pressure of 3.5 MPa, and it is coupled with aregenerator that operates at 550° C. under atmospheric pressure. Thecatalyst is drawn off at regular time intervals at the bottom of thereactor and transferred to the regenerator, from which it is sent to thetop of the reactor, whereby the transfers were made through bufferlocks. At the outlet of the reactor, the unconverted ethylene isfractionated in a first distillation column and recycled. A seconddistillation column separates the propylene and the unconverted C₄hydrocarbons that are also recycled. The composition of the metathesiseffluent is indicated in Table 1, flow 8.

[0063] The overall balance of the transformation is therefore found tobe as follows. Per 100 parts by weight (pp) of the C₄ fraction that hasleft the steam-cracking device, 1.6 pp of hydrogen and 44 pp of ethyleneare consumed, and 118 pp of propylene is produced. At the steam-crackingdevice from which is obtained the treated C₄ fraction, this balancetherefore represents a modest ethylene consumption and a significantadditional propylene production.

[0064] The advantage of this process is therefore to produce in a veryselective way a polymerization-quality propylene in particular with themetathesis of a butene-2 feedstock that contains only small amounts ofbutene-1 and isobutene, a feedstock that is obtained byhydroisomerization and skeletal isomerization of a C₄ fraction, whichmakes it possible to enhance all of the olefins of this fraction interms of propylene. TABLE 1 1 1 + 6 3 5 4 Flow No C4 Hydro- Hydro- 4Isobutene 6 9 Isobutene 4 + 7 8 (FIG. 1) Feed- IsomerizationIsomerization Stabilization Column Butenes Butenes Column MetathesisMetathesis (kg/h) stock Inlet Outlet Outlet Head Recycling Purge BottomInlet Outlet (C3 + C3) 10 10 41 = MAPD 31 31 Isobutane 446 6424 64246424 6424 5978 446 n-Butane 545 545 988 988 988 988 988 Isobutene 57419588 9588 9588 9588 3847 287 Butene-1 3407 6455 1423 1423 1312 3048 238111 111 89 Butenes-2 2250 4990 18095 18095 2740 198 18095 18095 1810Butadiene- 8095 8095 1,3 BBV 104 104 Hydrogen 343 26 Methane 197 197Ethylene 9048 845 Propylene 24428 Heavy 504 504 504 504 38 504 504 586Products TOTAL 20629 37286 37286 37022 17324 16117 1207 19698 2874628746

[0065] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples. Also, the preceding specific embodiments are to be construedas merely illustrative, and not limitative of the remainder of thedisclosure in any way whatsoever.

[0066] The entire disclosure of all applications, patents andpublications, cited above and below, and of corresponding Frenchapplication 99/16.506, are hereby incorporated by reference.

[0067] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. Process for converting into propylene an olefinic C₄ fraction, whereby said fraction comprises diolefins, primarily butadiene-1,3, butene-1, butene-2, isobutene and acetylenic impurities, and whereby said process comprises the following stages that take place successively: 1) the selective hydrogenation of diolefins and acetylenic impurities with isomerization of butene-1 into butenes-2, carried out in a reactor, in the presence of a catalyst, in order to obtain an effluent that contains for the most part butenes-2 and isobutene, and that contains virtually no diolefins or acetylenic compounds; 2) the separation by distillation of a top fraction that contains for the most part isobutene and unconverted butene-1 in the first stage, and a bottom fraction that contains essentially butene-2 and butane; and 4) the metathesis of the butenes-2 fraction that is obtained from stage 2 with the ethylene so as to obtain an effluent that contains propylene, whereby the metathesis is followed by a separation of the propylene; whereby said process also comprises a stage 3 of skeletal isomerization of the isobutene into n-butenes in the top fraction, with recycling of at least a portion of the effluent in stage
 1. 2. Process according to claim 1, characterized in that stage 1 is carried out by running said fraction in the liquid phase over a catalyst that comprises at least one metal that is selected from the group that is formed by nickel, palladium and platinum, deposited on a substrate, at a temperature of 0 to 200° C., a pressure of 0.1 to 5 MPa, a volumetric flow rate of 0.5 to 10 h⁻1, with an H₂/diolefin molar ratio of 0.5 to
 5. 3. Process according to claim 1 or 2, wherein the catalyst of stage 1 contains 0.05 to 10% by weight of sulfur.
 4. Process according to one of claims 1 to 3, wherein the catalyst of stage 1 was treated, before being loaded into the hydrogenation reactor, by at least one sulfur-containing compound that is diluted in a solvent, and wherein the catalyst that is obtained and that contains 0.05 to 10% by weight of sulfur is loaded into a reactor and activated under a neutral atmosphere or a reducing atmosphere at a temperature of 20 to 300° C., a pressure of 0.1 to 5 MPa and a VVH of 50 to 600 h⁻1, and wherein the feedstock is brought into contact with said activated catalyst.
 5. Process according to one of claims 3 and 4, wherein the catalyst of stage 1 consists of palladium that is deposited on alumina and sulfur.
 6. Process according to one of claims 1 to 5, wherein the isomerization of butene-1 into butene-2 that is carried out in stage 1 and the distillation of stage 2 are joined in a single stage that causes a reactive distillation column that includes on the inside or outside an isomerization catalyst as described for stage 1 to take effect.
 7. Process according to one of claims 1 to 6, wherein in stage 3, the skeletal isomerization of isobutene into n-butenes, with recycling of the effluent in stage 1, is carried out with a catalyst that comprises alumina and titanium, at a temperature of 300° C. to 570° C., a pressure of 0.1 to 1 MPa, at a volumetric flow rate of 0.1 to 10 h⁻1, and in the presence of water injection, whereby the injected water/olefinic hydrocarbons molar ratio is 0.1 to
 10. 8. Process according to claim 7, wherein the skeletal-isomerization catalyst that is used in stage 3 contains alumina and 0.03 to 0.6% by weight of titanium and 0.05 to 5% by weight of an oxide of an element of group IIIA.
 9. Process according to one of claims 1 to 8, wherein the metathesis is carried out in stage 4 in the presence of a catalyst that comprises at least one rhenium oxide that is deposited on a substrate at a temperature of 0 to 200° C., and at a pressure that is at least equal to the vapor pressure of the reaction mixture at the reaction temperature.
 10. Process according to claim 9, wherein said catalyst contains rhenium oxide at a rate of 0.01 to 20% by weight expressed in metallic rhenium, deposited on a substrate that contains at least 75% by weight of alumina and 0.01 to 30% by weight of at least one oxide of a metal that is selected from the group that is formed by niobium and tantalum.
 11. Process according to one of claims 8 or 9, wherein the metathesis is carried out with a moving-bed catalyst.
 12. Process according to one of claims 1 to 11, wherein the C₄ fraction that is to be treated is a steam-cracking fraction, and the ethylene that is used in the metathesis stage is obtained from the steam-cracking operation.
 13. Process according to one of claims 1 to 12, wherein the bottom fraction of distillation stage 2 contains at most 1% by weight of isobutene and at most 1% by weight of butene-1.
 14. Installation for the conversion of an olefinic C₄ fraction into isobutene and into propylene, successively comprising: 1) a selective hydrogenation zone 1 with isomerization of butene-1 into butene-2, whereby said zone comprises at least one means 1 for introducing the C₄ fraction that is to be converted, at least one means 3 for the output of the effluent and at least one means 2 for the introduction of hydrogen, whereby said zone also comprises at least one catalyst bed; 2) a zone 2 for separation that comprises at least one means 3 for introducing the effluent that is obtained from zone 1, at least one means 5 for the output of isobutene and butene-1, at least one means 4 for the output of butene-2 and n-butane; and 4) a metathesis zone 4 that contains at least one catalyst bed and that comprises at least one means 4 for introducing the effluent that is obtained from zone 2, at least one means 7 for introducing ethylene and at least one means 8 for the output of propylene, whereby said installation also comprises a skeletal-isomerization zone 3 that comprises at least one means 5 for introducing the effluent that is obtained from zone 2, at least one means 6 for recycling from the outlet of zone 3 to the inlet of zone 1 and at least one means 9 for purging the optionally present isobutane, whereby said zone also comprises at least one catalyst bed that preferably comprises alumina and titanium.
 15. Installation according to claim 14, wherein the metathesis zone contains a catalyst moving bed.
 16. Installation according to one of claims 14 and 15, wherein the means for introducing the C₄ fraction that is to be converted is connected to a steam-cracking zone and wherein the means for introducing ethylene into the metathesis zone is connected to said steam-cracking zone. 